In typical papermaking processes, a general correlation exists between fiber coarseness and softness or handfeel of the resulting paper product.
Expensive high quality fibers such as bleached northern softwood kraft fibers are fine, flexible and are used to produce soft, desirable tissue products. In contrast, mechanical pulping of softwoods produces high-yield, coarse, stiff fibers typically used to make newsprint.
Newspapers contain a preponderance of coarse, high yield fibers, typically stone groundwood (SGW), thermomechanical pulp (TMP), and/or chemithermomechanical pulp (CTMP) fibers. Such coarse newsprint fibers are usually highly refined to cause fractures and fibrillations which aid in imparting strength to the resulting newsprint. Such refining changes the freeness of the coarse fiber from "high" freeness fibers to "low" freeness fibers. If such refined, high-yield, coarse, mechanically pulped fibers were used in a tissue making process the resulting sheet is not soft, and therefore much less desirable as a tissue product.
A recent thorough discussion of the relationship between tissue softness and fiber coarseness is contained in Canadian Patent No. 2,076,615. Attempts to produce soft tissue or towel type sanitary paper products from a majority of high yield fibers such as CTMP, TMP or SGW pulp have not been successful. Likewise, producing soft tissue and towel products by recycling old newspapers has not been very successful partially because the predominant fiber in newsprint or in old newspapers are low freeness, coarse, high yield fibers.
Other complicating factors in producing soft tissue and towel products from recycled newspapers are problems with papermachine operation due to poor drainage of low freeness fibers and problems with fines and other substances that accumulate in the papermachine water system (whitewater). These materials make it difficult to crepe the tissue sheet from the Yankee drying cylinder, and therefore necessitate operating the papermachine at conditions which do not promote maximum softness.
Conventional recycling of newspapers/newsprint to obtain fibers comparable to the type of fibers used to originally make the newsprint is known in the art as "deinking" and typically involves pulping, washing (usually with surfactants), screening, centrifugal cleaning, solubilizing insoluble contaminants (usually by strong caustic treatments), washing and bleaching of the fibers to counteract the yellowing effects of caustic treatments.
The first step in conventional recycling of old newspapers is to separate the paper into individual fibers in water to form a pulp slurry. Surfactants and caustic are added to facilitate the solubilization and separation of contaminants from the fibers. This is followed by removing inks and contaminants from the fibers by a combination of various process steps such as screening, centrifugal cleaning, washing, flotation and the like. The screening and centrifugal cleaning steps remove large contaminants such as paper clips, staples, plastics, etc. The primary purpose of washing and flotation steps is to suspend contaminants in the water and to remove the contaminants from the fibers.
When caustic is used to facilitate contaminant removal, some yellowing of the fibers unfortunately occurs due to the caustic treatment. After or during caustic treatment and washing, the fibers are usually bleached (e.g. with hydrogen peroxide) to counteract the yellowing effect of caustic or to produce better fibers having higher brightness than the fibers in the original waste paper. Cleaned, decontaminated, and bleached fibers are usually blended with virgin fibers and then used in a papermaking process for which the fibers properties are suitable. Because the starting fibers are newsprint type fibers (i.e., coarse, low freeness and low brightness fibers) such recycled fibers are most often reused for making blank newsprint. Their high coarseness and low freeness render them unsuitable for making soft tissue products unless blended with a majority of higher quality fibers such as bleached northern softwood kraft pulp.
Conventional pulping of used newspaper to obtain recycled newsprint fiber is usually done in a high attrition pulper at a consistency of 4-8% and at 90.degree. F.-160.degree. F. for 20 minutes to 60 minutes, depending on the exact type of waste paper being processed. Caustic soda or other alkaline substances such as sodium silicate are commonly used to raise the pH of the pulp slurry to pH 9-10 to aid in separating fibers (defibering) and also to loosen the inks and separate dirt from the fiber. At an alkaline pH vegetable oils in the inks are saponified while mineral oils are emulsified by the combination of alkaline pH, soap, and surfactants, all of which enhance the removal of oils during washing. A surfactant deinking aid (for higher pH ranges) is usually added to further help separate inks from fiber.
The caustic step in recycling processes of old newsprint to obtain well cleaned quality fibers causes swelling of the fibers, and usually solubilizes many components. In addition to saponifying vegetable based printing oils, caustic also saponifies natural organic acids typically present in old newspapers to produce the corresponding soaps of the saponifiable materials. The saponified vegetable oils and organic acids so formed aid in removal of other contaminants from the fibers, such as non-saponifiable printing oils (mineral oil). These substances are subsequently removed from the fibers by washing and/or flotation after the caustic treatment.
A major recycler of old newspapers, Garden State Paper, in recent journal articles, one entitled "The Big "'D": Getting Rid of the Ink in Recycled Fiber, appearing in the journal Paper Age, 1991 Recycling Annual, at pages 23 and 50 and the other article entitled "Recycling From the Newsprint Perspective, at pages 9, 12 and 13 of the same 1991 Recycling Annual, (Paper Age, 1991 Recycling Annual) describes its newsprint recycling and deinking processes cleaning and screening followed by a series of 3 washings facilitated by the addition of chemicals to emulsify the printing oils and resins. Again the aim of this process is to remove printing ink constituents, including oils, as completely as possible. This is especially important because the recycled newsprint fiber is made into blank newsprint paper which would not have adequate brightness or strength without removing the ink constituents.
There is a long felt and unmet need for a soft paper product made from high-yield, coarse, fibers. There is also a need for an economical and practical process of treating high-yield, coarse fibers so they are suitable for making soft paper products. This need also extends to a process for treating newspapers/newsprint fibers so they are suitable for making soft paper products as well as soft paper products containing such treated fibers.